Having participated in 10 jams, I'd say unless the jam specifically points to the theme as optional, it is the entire point of the jam (other than the time limit).
It is a prompt to be responded to. It gives cohesion to what would otherwise be a bunch of random games, challenges developers to get outside their comfort zone, and provides the primary source of competition among devs to craft the most clever interpretation of the theme in so little time.
Nonetheless, it's still a great achievement to finish a game during the period!